[insanitybit]

The ability to cancel an operation without talking to the operating system requires that your program has yield points. That yielding is what allows another part of the program to take control and say "OK, I'm done with you now, no need to finish".

Yes, the problem is that your thread would continue to perform work even if you stopped waiting on it.

[vc8f6vVV]

Maybe I don't understand the complexity, but in good old Ruby I can easily stop a thread if I don't need result anymore. No async needed and no yield points necessary. Doesn't it apply to Rust too?

[insanitybit]

I assume that Ruby does in fact have yield points in some form, such as a global lock. Killing a thread is only possible (for a pthread) via the `pthread_cancel` API. That API is very dangerous and is generally not something you'd ever want to use manually - the thread will not clean up any memory or other resources, any shared memory is left in a tricky state.

To gracefully shut a thread down you need yielding of some kind.